The present invention relates, in general, to solid state sensors, and more particularly, to solid state sensors for detecting a presence of reprographic media in a printer using reflected electromagnetic radiation.
Optical sensors, including combinations of light emitting diodes (LEDs) and photodiode detectors are used to detect paper in printing machines such as xerographic copiers and laser printers. Currently available systems use a single LED aimed at a portion of a paper path where a sheet of paper travels. Usually only a single detector is used to detect a change in the quantity of light reflected from or transmitted through the sheet of paper. An example of such a system is shown in U.S. Pat. No. 4,983,854 issued to Toshiaki Mizuno et al on Jan. 8, 1991.
Since the single detector only responds to a change in reflectivity, the reflectivity contrast between a "paper present" and a "paper absent" condition must be significant. While such systems are adequate for detecting white paper against a dark background, they are less reliable when detecting dark or colored paper. Moreover, it is virtually impossible to detect a transparent sheet of reprographic media, commonly used for overhead projection. One solution to these difficulties is to add a leader strip of solid paper to one edge of the transparent sheet, increasing cost of the media.
What is needed is a reprographic media detector that reliably detects a sheet of media regardless of reflectivity, color, or transparency.